Strain gauge with thermal current



9, 1945. c, G 22,589 I STRAINGAU-QE WITH THERMAL CURRENT CONTROL Original Filed Sept. 16, 1939 \NVENTOR RTHUR C. RU B fgl/ 1 a. V

A ORNEY Reiuued Jan. 9, 1945 w scram GAUGE, wrrn THERMAL'CUBRENT coNrrnoL -Arthur C. Ruge, Cambridge, Mass., assignor to The Baldwin Locomotive Works, a corporation of Pennsylvania.

Original No. 2,334,843, dated November 23, 1943, Serial No. 453,505, August 4, 1942, which is-a division of Serial No. 295,207, September 16, 1939. Application for reissue October 4, 1944, 7

Serial No. 557,115

4 Claims. (Cl. 201--63.)

Thisinvention relates generally to strain gauges and more particularly to a gauge of the yp e ploying a continuous solid filament of electrical conducting material bonded throughout its effective length to the surface of a member under test and whose electrical resistance varies in response to strain, this application being a division of my copending application Serial No. 295,207, filed September 16, 1939.

Ina gauge of the foregoing type an electrical current is passed through the wire filament and as aresult thermal currents may arise which will distort the accuracy or the gauge, this possibility of distortion being aggravated by the extremely small magnitude of the resistance change in the gauge in response to variations in strain.

It is an object of my invention to provide an improved electrical strain gauge that eliminates or minimizes thermal current. A further object is to provide an improved electrical strain gauge adapted to eliminate thermal currents in a simple, direct and effective manner, is economical in construction and operation and is compact and rugged.

Other objects and advantages will be more apparent to those skilled in the art from the following description of the accompanying drawing which is a perspective of my improved gauge applied to a test specimen.

A strain responsive filament I04a of electrical conducting material such as disclosed in my copending-applicationis bonded throughout its effective length to the surface of a member I04d subject to strain. Short pieces Illlb or material identical to leads I04 are interposed between the strain filament I04a and leads I04, I04, the pieces Il4b being very small in cross-sectional area relative to their length. The entire filament and lead elements are cemented, as indicated by the speckled area I05, to a carrying medium or to a specimen and suitably insulated therefrom if necessary. The material of which filament I 04a is made and also its various characteristics are disclosed in my said copending application and hence need not be further described in this application except to point out that the filament is preferably of the approximate order of .001" to- .003" in diameter thus showing the extremely minute nature of the electrical element.

If the leads I04. I04 were connected directly to filament 104a at points I040 and should one filament arising from a temperature difference at these Junctures. It is possible to eliminate thermal current effects from the results of electrical resistance'measurements by methods well-known in precise resistance work but such methodsare not convenient in strain measurement work. For example, if one terminal lead becomes warmed. more than the other, even if the temperature in the specimen is uniform, heat will iiow down that lead to warm both the lead and its junction with filament Il4a. This may happen without the observer's knowledge thus causing him to make an error in strain reading. However, in my improved arrangement ifheat is applied to one of the leads I04 it will have to flow through pieces Il'lb before reaching the thermal junction Iilc. Since the piece I04b is small in area and relatively long, as compared to the length of filament I040, substantially all heat applied tolead I04 will flow into the surrounding structure or -specimen "4:! before the heat reaches the thermal junction "40. Hence the strain filament I04a will be unaffected by any thermal currents. The thermal dissipating pieces Il4b may be made of material different from that of the leads I04 pro ded It has a zero or negligible thermal-electrica eifect'against the leads. For example, the

leads can be copper and the pieces I04b can be Manganin, while the strain sensitive filament can be any desired material such as Elinvar or Ad- Vance. It is not important whether the pieces I04b possess strain sensitivity, the choice here depending upon what overall characteristics are desired.

If the surface of specimen Him is not at a uniform temperature all over, then it is desirable of these junctures be at a different temperature virtue of thermal currents between the leads and 55 to have the two iunctions I040 close together. I have used strain gauges embodying my invention, but without pieces I04b, in circuits which were very difilcuitto deal with due to thermal currents. After adding pieces I040 the thermal currents dropped to a negligible value even under adverse temperature conditions. For purposes of illustration, Advance wire may be used as the strain filament M41: as this has a high thermal electric eil'ect against copper. The pieces I041) may be copper wire of a length about A and .002" diameter while the leads I04 may be copper about .04" diameter. With such an arrangement the lead wires could be purposely heated far more than they would be normally heated and yet the .002" wires would add so little resistance that the sensitivity of the usual Wheatstone measuring bridge would not be measurably affected.

From the foregoing disclosure it is seen that I have provided an extremely simple and highly eflective arransemnntfor eliminating or minimizing thermal currents in a gauge which has a high degree of sensitivity, accuracy and responsiveness without in any way sacrificing these very desirable qualities.

It will or course be understood that various changes in details or construction and arrangement of parts may be made by those skilled in the art without departing from the spirit of the invention as set forth in the appended claims.

I claim:

1. A strain gauge comprising a filament of continuous solid electrical conducting material whose electrical resistance. varies in accordance with changes of strain therein, means for bonding said filament throughout its eifective length to a member subject to strain whereby the strain of the filament follows that 01' said member. to efie ct a corresponding change or filament resistance, leads for said filament, and other filamentsconnected to the ends of said bonded filament andtosaid leads and having a. cross-sectional area relative to that of said leads for substantially preventing thermal current eflects in said bonded filament.

.2. A strain gauge comprising a filament of cone tinuous solid electrical conducting material whose electrical resistance varies in accordance with changes oi! strain therein, means for bonding said filament throughout its effective length to a member subject to strain whereby the strain'ot the filament follows that 01 said member to efiecta corresponding change of filament resistance, leads for said filament, other filaments connected to the ends of said strain filament and to saidleads and having a smaller cross-sectional area than said leads thereby to prevent thermal current effects, and means for bonding said thermal current filaments through their length to the member subject to strain.

3. A strain gauge comprising a filament of continuous solid electrical conducting material whose electrical resistance varies in accordance with changes or strain therein, means for bonding said filament throughout its effective length to a member subject to strain whereby the strain of the filament follows that of said member to efiect a corresponding change of filament resistance, leads for said filament, and wires of smaller size than said leads 'disposed'between and connected to the leads and filament.

4. The combination set forth in claim 3 further characterized in that said wires and leads are commonly bonded wlth said filament to a member subject to strain.

ARTHUR C. RUGE. 

